0.5.0: Calls API

• Merged PR #13: Use marshalling for function calls. This way functions
  can have their arguments converted from and return values converted
  to JSON primitives
  (@thekid)
• Merged PR #12: Wrap function calling in Calls API. This simplifies
  function calling by handling JSON de- and encoding as well as supplying
  a default error handling mechanism
  (@thekid)